Evidence that the Crystalline Cores of Uplifts Adjacent to the Powder River Basin Were Breached During Paleocene Time

Sandstones in the upper part of the Paleocene Tongue River Member of the Fort Union Formation in the Powder River basin are dominantly sublitharenite. These rocks contain abundant rock fragments of non-ferroan calcite, dolomite, chert, and foliated tine-grained metamorphic rock (phyllite). The carbonate and chert rock fragments were probably eroded from Paleozoic carbonate sequences flanking the Bighorn Mountains or the Black Hills. The phyllffic rock fragments indicate that the crystalline cores of these uplifts were exposed during Paleocene time, which is earlier during the Laramide Orogeny than has been previously demonstrated.